Why is it called a literary cycle?

Question

The word cycle itself is simple enough when used to mean something that returns to its beginning, something that loops. But how did it also gain the meaning of a literary cycle, defined as a collection of stories with the same set of characters? There doesn't seem to be any obvious link between the two meanings, and nothing I can find gives an etymology of the latter meaning in particular.

Answer 1

The Oxford English Dictionary has the following definition for "cycle":

A series of poems or prose romances, collected round or relating to a central event or epoch of mythic history and forming a continuous narrative; as the Arthurian cycle. Also transf.
_{Originally used in the Epic cycle [Greek ὁ (ἐπικὸς) κύκλος], the series of epic poems written by later poets (Cyclic poets) to complete Homer, and presenting (with the Iliad and Odyssey) a continuous history of the Trojan war and of all the heroes engaged in it.}

I found some more information about this in the 1911 Encyclopædia Britannica:

The expression “epic cycle” in the sense of a poetical collection does not occur before the Christian era; the word κύκλος (“cycle,” “circle”) is used of a special kind of short poem and also of a prose abstract of mythological history; the adjective has the general sense of “hackneyed,” “conventional,” and is applied contemptuously (by Callimachus and Horace) to a particular Alexandrian school of poetry.

Beyond that, one source says that "[t]heir reason for using the term 'cyclic' (if in fact they did) remains obscure", so I'm not sure there is a definite answer, ultimately.

This sense of the word in English dates at least as far back as 1835:

They..formed the basis or nucleus of the epic cycle.
_{A History of Greece, Volume 1}

The adjective form dates back to a1822:

They are the episodes of that cyclic poem written by Time upon the memories of men.
_{Prose works}

Answer 2

Inspired by Laurel's answer, I tried to find the earliest reference I could to literary cycles in English, and I found this excerpt from 1749 (!) that seems to hint at an etymology:

The principal Kind of Cyclic Poem was, when the Poet carried his Subject from one fixed Period of Time to another, as from the Beginning of the World to the Trojan War, and connected all the Events together in a continued Train. It is in this last Sense that Ovid calls his Metamorphoses perpetuum carmen, all the Parts being connected together by the most natural and easy Transitions...

So, the idea seems to be that a poem cycle is "cyclical" not in its content, but in its structure - the end of each work is the beginning of the next. Which makes a lot of sense to me: it's like the idea of the cyclical nature of time, recognizing the repeating annual patterns of sun and warmth and growth, though obviously the content of each year is unique!

I suppose this concept was later expanded to any collection of works that form a greater whole, particularly those that, like the original epic cycle, follow the same characters through multiple stories. By the time you get to the idea of "song cycles," the connection between the works is the main thing, and the idea of circularity has been lost entirely.